Biosensor Recognition of Thyroid-Disrupting Chemicals Using Transport Proteins

In silico models for the screening of human transthyretin disruptors

The use of New Approach Methodologies (NAMs), such as Quantitative Structure-Activity Relationship (QSAR) models, is highly recommended by international regulations to speed up hazard and risk assessment of Endocrine Disruptors, which are known to be linked to a wide spectrum of severe diseases on humans and wildlife. A very sensitive target for these chemicals is the thyroid hormone system, which plays a key role in regulating metabolic and cognitive functions. Several chemicals have been demonstrated to compete with the thyroid hormone thyroxine (T4) for binding to human thyroid hormone distributor protein transthyretin (hTTR). In this work, we generated three new datasets composed by T4-hTTR competing potencies of more than 200 heterogeneous chemicals measured by three different in vitro assays. These datasets were used for the development of new regression QSAR models. The best models were thoroughly validated by internal and external validation procedures. The mechanistic interpretation of the selected molecular descriptors provided information on structural features which are relevant to characterise hTTR binders, such as the presence of hydroxylated and halogenated aromatic rings. PCA analysis was used to rank the studied chemicals according to their increasing T4-hTTR competing potency. Hydroxylated and halogenated bicyclic aromatic compounds are ranked as the strongest hTTR binders. The new QSARs are useful to screen potential Thyroid Hormone System-Disrupting Chemicals (THSDCs), and to support the identification of sustainable alternatives to hazardous chemicals.

Optimized methods for measuring competitive binding of chemical substances to thyroid hormone distributor proteins transthyretin and thyroxine binding globulin

Transthyretin (TTR) and thyroxine-binding globulin (TBG) are two major thyroid hormone (TH) distributor proteins in human plasma, playing important roles in stabilizing the TH levels in plasma, delivery of TH to target tissues, and trans-barrier transport. Binding of xenobiotics to these distributor proteins can potentially affect all these three important roles of distributor proteins. Therefore, fast and cost-effective experimental methods are required for both TTR and TBG to screen both existing and new chemicals for their potential binding. In the present study, the TTR-binding assay was therefore simplified, optimized and pre-validated, while a new TBG-binding assay was developed based on fluorescence polarization as a readout. Seven model compounds (including positive and negative controls) were tested in the pre-validation study of the optimized TTR-binding assay and in the newly developed TBG-binding assay. The dissociation constants of the natural ligand (thyroxine, T4) and potential competitors were determined and compared between two distributor proteins, showing striking differences for perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

Screening the ToxCast Chemical Libraries for Binding to Transthyretin

Transthyretin (TTR) is one of the serum binding proteins responsible for transport of thyroid hormones (TH) to target tissue and for maintaining the balance of available TH. Chemical binding to TTR and subsequent displacement of TH has been identified as an end point in screening chemicals for potential disruption of the thyroid system. To address the lack of data regarding chemicals binding to TTR, we optimized an in vitro assay utilizing the fluorescent probe 8-anilino-1-napthalenesulfonic acid (ANSA) and the human protein TTR to screen over 1500 chemicals from the U.S. EPA's ToxCast ph1_v2, ph2, and e1k libraries utilizing a tiered approach. Testing of a single high concentration (target 100 μM) resulted in 888 chemicals with 20% or greater activity based on displacement of ANSA from TTR. Of these, 282 chemicals had activity of 85% or greater and were further tested in 12-point concentration-response with target concentrations ranging from 0.015 to 100 μM. An EC50 was obtained for 276 of these 301 chemicals. To date, this is the largest set of chemicals screened for binding to TTR. Utilization of this assay is a significant contribution toward expanding the suite of in vitro assays used to identify chemicals with the potential to disrupt thyroid hormone homeostasis.

Identification of polar bioactive substances in the Upper Rhine using effect-directed analysis

Effect-Directed Analysis (EDA) was used to identify bioactive compounds in surface and well water from the Upper Rhine, and to evaluate their properties against the criteria set for Persistent, Mobile and Toxic (PMT) and very persistent and very mobile (vPvM) substances. A multi-layered solid-phase extraction was implemented to enrich a broad range of polar substances from the collected samples. The extracts were fractionated into 108 fractions and tested in the transthyretin (TTR)-binding assay measuring displacement of fluorescently labeled thyroxine (FITC-T4 TTR-binding assay) and the Aliivibrio fischeri bioluminescence (AFB) bioassay. Bioactive fractions guided the identification strategy using high-resolution mass spectrometry. Chemical features were systematically annotated using library databases and suspect lists, incorporating an automated assessment of the quality of each annotation. Based on this assessment, each chemical feature was assigned a specific identification confidence level. Identification of bioactive compounds was facilitated by using bioassay specific suspect lists that were extracted from an in-house developed database of positive and negative TTR-binding compounds and from a recently published database of active inhibitors of AFB. This resulted in the identification and confirmation of ten bioactive substances, including four evaluated as PMT and vPvM substances (diclofenac, trifloxystrobin acid, 6:2 FTSA and PFOA), and one as a potential PMT substance (4-aminoazobenzene). This study demonstrates the effectiveness of EDA in the identification of PMT/vPvM substances in the aquatic environment, facilitating their prioritization for comprehensive environmental risk assessment and possible regulation.

Binding of common organic UV-filters to the thyroid hormone transport protein transthyretin using in vitro and in silico studies: Potential implications in health

Several anthropogenic contaminants have been identified as competing with the thyroid hormone thyroxine (T₄) for binding to transport proteins as transthyretin (TTR). This binding can potentially create toxicity mechanisms posing a threat to human health. Many organic UV filters (UVFs) and paraben preservatives (PBs), widely used in personal care products, are chemicals of emerging concern due to their adverse effects as potential thyroid-disrupting compounds. Recently, organic UVFs have been found in paired maternal and fetal samples and PBs have been detected in placenta, which opens the possibility of the involvement of TTR in the transfer of these chemicals across physiological barriers. We aimed to investigate a discrete set of organic UVFs and PBs to identify novel TTR binders. The binding affinities of target UVFs towards TTR were evaluated using in vitro T₄ competitive binding assays. The ligand-TTR affinities were determined by isothermal titration calorimetry (ITC) and compared with known TTR ligands. In parallel, computational studies were used to predict the 3-D structures of the binding modes of these chemicals to TTR. Some organic UVFs, compounds 2,2',4,4'-tetrahydroxybenzophenone (BP2, Kd = 0.43 μM); 2,4-dihydroxybenzophenone (BP1, Kd = 0.60 μM); 4,4'-dihydroxybenzophenone (4DHB, Kd = 0.83 μM), and 4-hydroxybenzophenone (4HB, Kd = 0.93 μM), were found to display a high affinity to TTR, being BP2 the strongest TTR binder (ΔH = -14.93 Kcal/mol). Finally, a correlation was found between the experimental ITC data and the TTR-ligand docking scores obtained by computational studies. The approach integrating in vitro assays and in silico methods constituted a useful tool to find TTR binders among common organic UVFs. Further studies on the involvement of the transporter protein TTR in assisting the transplacental transfer of these chemicals across physiological barriers and the long-term consequences of prenatal exposure to them should be pursued.

Towards regulation of Endocrine Disrupting chemicals (EDCs) in water resources using bioassays - A guide to developing a testing strategy

Endocrine disrupting chemicals (EDCs) are found in every environmental medium and are chemically diverse. Their presence in water resources can negatively impact the health of both human and wildlife. Currently, there are no mandatory screening mandates or regulations for EDC levels in complex water samples globally. Bioassays, which allow quantifying in vivo or in vitro biological effects of chemicals are used commonly to assess acute toxicity in water. The existing OECD framework to identify single-compound EDCs offers a set of bioassays that are validated for the Estrogen-, Androgen-, and Thyroid hormones, and for Steroidogenesis pathways (EATS). In this review, we discussed bioassays that could be potentially used to screen EDCs in water resources, including in vivo and in vitro bioassays using invertebrates, fish, amphibians, and/or mammalians species. Strengths and weaknesses of samples preparation for complex water samples are discussed. We also review how to calculate the Effect-Based Trigger values, which could serve as thresholds to determine if a given water sample poses a risk based on existing quality standards. This work aims to assist governments and regulatory agencies in developing a testing strategy towards regulation of EDCs in water resources worldwide. The main recommendations include 1) opting for internationally validated cell reporter in vitro bioassays to reduce animal use & cost; 2) testing for cell viability (a critical parameter) when using in vitro bioassays; and 3) evaluating the recovery of the water sample preparation method selected. This review also highlights future research avenues for the EDC screening revolution (e.g., 3D tissue culture, transgenic animals, OMICs, and Adverse Outcome Pathways (AOPs)).

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches

BACKGROUND

Extensive clinical and experimental research documents the potential for chemical disruption of thyroid hormone (TH) signaling through multiple molecular targets. Perturbation of TH signaling can lead to abnormal brain development, cognitive impairments, and other adverse outcomes in humans and wildlife. To increase chemical safety screening efficiency and reduce vertebrate animal testing, in vitro assays that identify chemical interactions with molecular targets of the thyroid system have been developed and implemented.

OBJECTIVES

We present an adverse outcome pathway (AOP) network to link data derived from in vitro assays that measure chemical interactions with thyroid molecular targets to downstream events and adverse outcomes traditionally derived from in vivo testing. We examine the role of new in vitro technologies, in the context of the AOP network, in facilitating consideration of several important regulatory and biological challenges in characterizing chemicals that exert effects through a thyroid mechanism.

DISCUSSION

There is a substantial body of knowledge describing chemical effects on molecular and physiological regulation of TH signaling and associated adverse outcomes. Until recently, few alternative nonanimal assays were available to interrogate chemical effects on TH signaling. With the development of these new tools, screening large libraries of chemicals for interactions with molecular targets of the thyroid is now possible. Measuring early chemical interactions with targets in the thyroid pathway provides a means of linking adverse outcomes, which may be influenced by many biological processes, to a thyroid mechanism. However, the use of in vitro assays beyond chemical screening is complicated by continuing limits in our knowledge of TH signaling in important life stages and tissues, such as during fetal brain development. Nonetheless, the thyroid AOP network provides an ideal tool for defining causal linkages of a chemical exerting thyroid-dependent effects and identifying research needs to quantify these effects in support of regulatory decision making. https://doi.org/10.1289/EHP5297.

An automated screening method for detecting compounds with goitrogenic activity using transgenic zebrafish embryos

The knowledge on environmentally relevant chemicals that may interfere with thyroid signaling is scarce. Here, we present a method for the screening of goitrogens, compounds that disrupt the thyroid gland function, based on the automatic orientation of zebrafish in a glass capillary and a subsequent imaging of reporter gene fluorescence in the thyroid gland of embryos of the transgenic zebrafish line tg(tg:mCherry). The tg(tg:mCherry) reporter gene indicates a compensatory upregulation of thyroglobulin, the thyroid hormone precursor, in response to inhibition of thyroid hormone synthesis. Fish embryos were exposed to a negative control compound (3,4-dichloroaniline), or a concentration series of known goitrogenic compounds (resorcinol, methimazole, potassium perchlorate, 6-propyl-2-thiouracil, ethylenethiourea, phloroglucinol, pyrazole) with maximum exposure concentration selected based on mortality and/or solubility. Exposure to 3,4-dichloroaniline decreased the fluorescence signal. All goitrogenic compounds exhibited clear concentration-dependent inductions of reporter fluorescence 1.4 to 2.6 fold above control levels. Concentration-response modelling was used to calculate goitrogenic potencies based on EC₅₀ values. The new automated method offers an efficient screening approach for goitrogenic activity.

Optimization of an in vitro assay methodology for competitive binding of thyroidogenic xenobiotics with thyroxine on human transthyretin and albumin

Thyroid hormones (THs) are involved in the regulation of many physiological processes in vertebrates. Competition for TH binding sites on serum transport proteins can interfere with delivery of THs to target tissues, and this is a potential mechanism of action of exogenous thyroidogenic substances. To date, detailed accounts of in vitro methods for competitive binding with THs on TH transport proteins (human or wildlife) are sparse. In the limited number of published studies on in vitro radio-labelled TH-TH transport protein interactions, method descriptions were brief and with insufficient details for successful replication. Furthermore, upon review of these methodologies, we identified several opportunities for optimization. The present study addresses the methodological deficiencies and describes, in detail, a fully optimized and validated competitive T4 radio-ligand binding assay with human transthyretin (TTR) and albumin (ALB).

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Significant improvements were made over previous methods, including better maintenance of protein stability and enhanced measurement of competition between different ligands.

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Sample size was reduced to allow use of small pre-packed size exclusion chromatography columns, which eliminates the rinsing step during the separation procedure.

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The assay was parameterized for use with T4 and human TTR and ALB.

Developmental toxicity and thyroid hormone-disrupting effects of 2,4-dichloro-6-nitrophenol in Chinese rare minnow (Gobiocypris rarus)

In the present study, to evaluate embryonic toxicity and the thyroid-disrupting effects of 2,4-dichloro-6-nitrophenol (DCNP), embryos and adults of Chinese rare minnow (Gobiocypris rarus) were exposed to 2, 20, and 200μg/L DCNP. In the embryo-larval assay, increased percentages of mortality and occurrence of malformations, decreased percentage of hatching, and decreased body length and body weight were observed after DCNP treatment. Moreover, the whole-body T3 levels were significantly increased at 20 and 200μg/L treatments, whereas the T4 levels were markedly decreased significantly (p<0.05) for all DCNP concentrations. In the adult fish assay, plasma T3 levels were significantly increased whereas plasma T4 levels were significantly reduced in the fish treated with 20 and 200μg/L (p<0.05). In addition, DCNP exposure significantly changed the transcription levels of thyroid system related genes, including dio1, dio2, me, nis, tr, and ttr. The increased responsiveness of thyroid hormone and mRNA expression levels of thyroid system related genes suggested that DCNP could disrupt the thyroid hormone synthesis and transport pathways. Therefore, our findings provide new insights of DCNP as a thyroid hormone-disrupting chemical.

Tetrabromobisphenol A Is an Efficient Stabilizer of the Transthyretin Tetramer

Amyloid formation of the human plasma protein transthyretin (TTR) is associated with several human disorders, including familial amyloidotic polyneuropathy (FAP) and senile systemic amyloidosis. Dissociation of TTR's native tetrameric assembly is the rate-limiting step in the conversion into amyloid, and this feature presents an avenue for intervention because binding of an appropriate ligand to the thyroxin hormone binding sites of TTR stabilizes the native tetrameric assembly and impairs conversion into amyloid. The desired features for an effective TTR stabilizer include high affinity for TTR, high selectivity in the presence of other proteins, no adverse side effects at the effective concentrations, and a long half-life in the body. In this study we show that the commonly used flame retardant tetrabromobisphenol A (TBBPA) efficiently stabilizes the tetrameric structure of TTR. The X-ray crystal structure shows TBBPA binding in the thyroxine binding pocket with bromines occupying two of the three halogen binding sites. Interestingly, TBBPA binds TTR with an extremely high selectivity in human plasma, and the effect is equal to the recently approved drug tafamidis and better than diflunisal, both of which have shown therapeutic effects against FAP. TBBPA consequently present an interesting scaffold for drug design. Its absorption, metabolism, and potential side-effects are discussed.

Organohalogens and their hydroxylated metabolites in the blood of pigs from an open waste dumping site in south India: association with hepatic cytochrome P450

The concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and their hydroxylated metabolites (OH-PCBs and OH-PBDEs) were measured in the blood of Eurasian wild pigs (Sus scrofa) from a municipal waste open dumping site (DS) and a reference site (RS) in South India. We showed that contamination with OH-PCBs was higher in female pigs from the DS than in all other adult pigs. The highest OH-PCB concentrations were found in piglets from the DS. Moreover, the hepatic expression levels of CYP1A and CYP2B were higher in piglets than in their dam, implying metabolism of PCBs by cytochrome P450 (CYP) enzymes. The OH-PCB congener profiles differed according to sex and collection sites, possibly because of variations in the expression levels of phase I and phase II enzymes among individual pigs, differences in the exposure sources, and maternal transfer of parent PCBs. The hepatic CYP1A expression levels were positively correlated with the blood concentrations of 4OH-CB107, 4OH-CB162, and 4OH-CB187, implying CYP1A-dependent formation of these OH-PCBs in the pig liver. We found no significant correlations between the blood concentrations of OH-PCBs and thyroid hormones (THs); however, the thyroxin (T4) levels were lower in pigs from the DS than in pigs from the RS. Our limited dataset suggest that induced CYP enzymes accelerate the metabolism of xenobiotics and endogenous molecules in pigs. Thus, besides parental compounds, the risk of hydroxylated metabolites entering wildlife and humans living in and around municipal open waste dumping sites should be considered.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Self-assembly of pyridine-modified lipoic Acid derivatives on gold and their interaction with thyroxine (t4)

Pyridyl derivatives of lipoic acid were prepared as ligands for the study of the interaction with thyroxine (T4). Thin self-assembled films of the ligands were prepared in 70% ethanol on gold and their interaction with T4 was studied by titration experiments in an aqueous buffer solution using Surface Plasmon Resonance (SPR). The thickness and refractive index of the ligand layers were calculated from SPR spectra recorded in two media, also allowing for surface coverage and the density of the layers to be estimated. Two ligands, a 4-pyridyl and a bis(2-hydroxyethyl) derivative of lipoic acid, were selected to investigate the feasibility for producing molecularly imprinted self-assembled layers on gold for T4. The methodology was to co-assemble T4 and the ligand onto the gold surface, elute the T4 from the layer under alkaline conditions, and study the rebinding of T4 to the layer. Multiple elution/rebinding cycles were conducted in different buffer solutions, and rebinding of T4 could be observed, with a moderate binding affinity that depended greatly on the solvent used. More optimal binding was observed in HBS buffer, and the affinity of the interaction could be slightly increased when the 4-pyridyl and bis(2-hydroxy-ethyl) derivatives of lipoic acid were combined in the imprinted layer.

Assessment of the binding of hydroxylated polybrominated diphenyl ethers to thyroid hormone transport proteins using a site-specific fluorescence probe

Polybrominated diphenyl ethers (PBDEs) have been shown to disrupt thyroid hormone (TH) functions on experimental animals, and one of the proposed disruption mechanisms is the competitive binding of PBDE metabolites to TH transport proteins. In this report, a nonradioactive, site-specific fluorescein-thyroxine (F-T4) conjugate was designed and synthesized as a fluorescence probe to study the binding interaction of hydroxylated PBDEs to thyroxine-binding globulin (TBG) and transthyretin (TTR), two major TH transport proteins in human plasma. Compared with free F-T4, the fluorescence intensity of TTR-bound conjugate was enhanced by as much as 2-fold, and the fluorescence polarization value of TBG-bound conjugate increased by more than 20-fold. These changes provide signal modulation mechanisms for F-T4 as a fluorescence probe. Based on fluorescence quantum yield and lifetime measurements, the fluorescence intensity enhancement was likely due to the elimination of intramolecular fluorescence quenching of fluorescein by T4 after F-T4 was bound to TTR. In circular dichroism and intrinsic tryptophan fluorescence measurements, F-T4 induced similar spectroscopic changes of the proteins as T4 did, suggesting that F-T4 bound to the proteins at the T4 binding site. By using F-T4 as the fluorescence probe in competitive binding assays, 11 OH-PBDEs with different levels of bromination and different hydroxylation positions were assessed for their binding affinity with TBG and TTR, respectively. The results indicate that the binding affinity generally increased with bromine number and OH position also played an important role. 3-OH-BDE-47 and 3'-OH-BDE-154 bound to TTR and TBG even stronger, respectively, than T4. With rising environmental level and high bioaccumulation capability, PBDEs have the potential to disrupt thyroid homeostasis by competitive binding with TH transport proteins.

Immunomagnetic microbeads for screening with flow cytometry and identification with nano-liquid chromatography mass spectrometry of ochratoxins in wheat and cereal

Multi-analyte binding assays for rapid screening of food contaminants require mass spectrometric identification of compound(s) in suspect samples. An optimal combination is obtained when the same bioreagents are used in both methods; moreover, miniaturisation is important because of the high costs of bioreagents. A concept is demonstrated using superparamagnetic microbeads coated with monoclonal antibodies (Mabs) in a novel direct inhibition flow cytometric immunoassay (FCIA) plus immunoaffinity isolation prior to identification by nano-liquid chromatography–quadrupole time-of-flight-mass spectrometry (nano-LC-Q-ToF-MS). As a model system, the mycotoxin ochratoxin A (OTA) and cross-reacting mycotoxin analogues were analysed in wheat and cereal samples, after a simple extraction, using the FCIA with anti-OTA Mabs. The limit of detection for OTA was 0.15 ng/g, which is far below the lowest maximum level of 3 ng/g established by the European Union. In the immunomagnetic isolation method, a 350-times-higher amount of beads was used to trap ochratoxins from sample extracts. Following a wash step, bound ochratoxins were dissociated from the Mabs using a small volume of acidified acetonitrile/water (2/8 v/v) prior to separation plus identification with nano-LC-Q-ToF-MS. In screened suspect naturally contaminated samples, OTA and its non-chlorinated analogue ochratoxin B were successfully identified by full scan accurate mass spectrometry as a proof of concept for identification of unknown but cross-reacting emerging mycotoxins. Due to the miniaturisation and bioaffinity isolation, this concept might be applicable for the use of other and more expensive bioreagents such as transport proteins and receptors for screening and identification of known and unknown (or masked) emerging food contaminants.

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Surface-plasmon-resonance-based chemical proteomics: efficient specific extraction and semiquantitative identification of cyclic nucleotide-binding proteins from cellular lysates by using a combination of surface plasmon resonance, sequential elution and liquid chromatography-tandem mass spectrometry

Chemical proteomics is a powerful methodology for identifying the cellular targets of small molecules, however, it is biased towards abundant proteins. Therefore, quantitative strategies are needed to distinguish between specific and nonspecific interactions. Here, we explore the potential of the combination of surface plasmon resonance (SPR) coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) as an alternative approach in chemical proteomics. We coupled cGMP molecules to the SPR chip, and monitored the binding and dissociation of proteins from a human lysate by using sequential elution steps and SPR. The eluted proteins were subsequently identified by LC-MS/MS. Our approach enabled the efficient and selective extraction of low-abundant cyclic-nucleotide-binding proteins such as cGMP-dependent protein kinase, and a quantitative assessment of the less- and nonspecific competitive binding proteins. The data show that SPR-based chemical proteomics is a promising alternative for the efficient specific extraction and quantitative identification of small-molecule-binding proteins from complex mixtures.

Metabolism of chlorinated biphenyls: use of 3,3'- and 3,5-dichlorobiphenyl as sole sources of carbon by natural species of Ralstonia and Pseudomonas

Ralstonia sp. SA-3, Ralstonia sp. SA-4 and Pseudomonas sp. SA-6 are natural strains with a novel capacity to utilize meta-substituted dichlorobiphenyls (diCBs) hitherto not known to serve as a sole source of carbon and energy for polychlorobiphenyl-degraders. In growth experiments, axenic cultures of isolates grew logarithmically on 3,3'-diCB with generation times that ranged insignificantly (t-test, P>0.05) from 30.4 to 33.8 h. Both 3-chlorobenzoate (3-CBA) and chloride produced as metabolites were recovered in non-stoichiometric quantities. The release of chloride by the cultures lagged substantially, indicating that the initial dioxygenase attack preceded cleavage of carbon-chloride bonds and that chloride must have been released from the chlorinated hydroxypentadienoate. In the case of 3,5-diCB, SA-3 and SA-6 metabolised this substrate primarily to 3,5-CBA. The lack of chloride in the culture media coupled with stoichiometric recovery of 3,5-CBA suggests that growth by these strains occurred predominantly at the expense of the unsubstituted phenyl ring. The unique metabolic properties of these three aerobic isolates point to their potential usefulness as seeds for bioremediation of PCBs polluted environments without the need for repeated inoculation or supplementation by a primary growth substrate such as biphenyl.

Enzymatic determination of BPA by means of tyrosinase immobilized on different carbon carriers

Different tyrosinase carbon paste modified electrodes to determine bisphenol A (BPA) concentration in aqueous solutions have been constructed. Variables examined were in the carbon paste composition and in particular: (i) the immobilized enzyme amount; (ii) the carbon type (powder, single or multi-walled nanotubes); (iii) the nature of the pasting oil (mineral oil, hexadecane and dodecane). For each biosensor type the amperometric response was evaluated with reference to the linear range and sensitivity. Constant reference has been made to the amperometric signals obtained, under the same experimental conditions, towards the catechol, a specific phenolic substrate for tyrosinase. The most efficient biosensors were those constructed by using the following composition for the carbon paste: 10% of tyrosinase, 45% of single wall carbon nanotubes (SWCN) and 45% of mineral oil. This biosensor formulation displayed the following electrochemical characteristics: a sensitivity equal to 138 microA/mM, LOD of 0.02 microM (based on three times the S/N ratio), linear range of 0.1-12 microM and response time of 6 min. This experimental work represents a first attempt at construction of a new carbon nanotube-tyrosinase based biosensor able to determine the concentration of BPA, one of the most ubiquitous and hazardous endocrine disruptors which can pollute the drinking and surface water, as well as many products of the food chain.

Integrating basic research on thyroid hormone action into screening and testing programs for thyroid disruptors

Thyroid hormone signaling is highly conserved among all the vertebrates, and appears to be present in some invertebrates. Both the components that comprise the system and its general role in development and physiology are evolutionarily conserved, although specific events regulated by thyroid hormones, such as amphibian metamorphosis, may differ among taxonomic groups. The articles in this issue review the thyroid systems of mammals (specifically humans and rodents), fish, amphibians, and birds, and the states of the assays and endpoints used to detect disruption of the thyroid system within a toxicological paradigm. It must be noted that while reptiles represent an enormously important group, they were excluded because there was not enough information in the literature on thyroid toxicology in reptiles at the time that this series of reviews was drafted. Each review highlights the best assays for current regulatory use and those that may be considered for development for future use and research. However, it is important to remember that thyroid research is moving ahead at a fast pace. New thyroid research will impact the design of future thyroid assays used for the detection of thyroid system disruption in ways that may not be anticipated at the time of this writing. Several new areas of exploration are discussed that reveal potential sites of disruption in the thyroid system, including (1) the importance of the neural drive for TSH upregulation, (2) thyroid hormone transport, including cellular transporters like monocarboxylate anion transporter 8 (MCT8) that can regulate thyroid hormone action at the cellular level, and thyroid hormone-binding proteins in the serum that have been shown to differentially bind to environmental chemicals (e.g., certain PCB congeners), and (3) the deiodinases as a target for disruption of thyroid hormone activity in the peripheral thyroid system. The review papers in this issue represent the current state of thyroid assays and endpoints for detection of chemicals that disrupt the thyroid system.

Spreeta-based biosensor assays for endocrine disruptors

The construction and performance of an automated low-cost Spreeta-based prototype biosensor system for the detection of endocrine disrupting chemicals (EDCs) is described. The system consists primarily of a Spreeta miniature liquid sensor incorporated into an aluminum flow cell holder, dedicated to support a Biacore chip frame, in combination with a simple pressurized air-driven fluid system. During the optimization, a monoclonal antibody (MAb)-based immunoassay for the estrogenic compound bisphenol A (BPA) was used as a model. After the optimization two thyroxine transport protein inhibition assays for thyroid endocrine disruptors were implemented. The average noise of the system for 1 min of baseline was 1.1 microRIU (refractive index units) and it could be operated in the range of 18-22 degrees C with a minimum baseline drift (5-10 microRIU/100 min). Optimum signal to noise ratio (S/N R) was obtained using a flow cell height of 100 microm and a flow rate of 180 microl/min. The sensitivity of the Spreeta-based biosensor inhibition assays implemented (50% inhibition concentration (IC50) of 30.2 nM for BPA using MAb12 and 12.3 and 11.6 nM for thyroxine (T4) using thyroxine-binding globulin (TBG) and recombinant transthyretin (rTTR), respectively) was comparable to the sensitivity previously obtained using a Biacore 3000 (IC50 of 39.9 nM for BPA and 8.6 and 13.7 nM, respectively, for T4). The results indicate that the alternative prototype system can be used in combination with ready-to-use biosensor chip surfaces and it is potentially a useful tool for the bioeffect-related screening of EDCs.

4-Hydroxy-PCB106 acts as a direct thyroid hormone receptor agonist in rat GH3 cells

Polychlorinated biphenyls (PCBs) may interfere with thyroid hormone (TH) action by interacting directly with the TH receptor (TR). We found that the hydroxylated PCB metabolite, 4-OH-CB106, bound to the human TRbeta1 and significantly elevated endogenous growth hormone (GH) expression in GH3 cells in a manner similar to that of T(3) itself. This effect was also observed using a consensus TH response element (TRE) in a luciferase expression system, and was blocked by a single base-pair substitution in this TRE. In addition, we found that 4-OH-CB106 did not alter the ability of TRbeta1 to physically interact with the TRE in the GH promoter, or with SRC1 or NCoR. These effects were directly parallel to effects of T(3), indicating that 4-OH-CB106 exerts a direct agonistic effect on the TRbeta1.